1. Field of the Invention
This invention relates to fluid flow measurement probes and, more specifically, it relates to flow measurement probes capable of measuring reactor coolant water flow in a cold leg pipe of a nuclear reactor system wherein the lack of elbows in the cold leg pipe precludes the use of elbow taps for flow measurement.
2. Description of the Prior Art
Water cooled nuclear reactors typically have at least two "cold leg" pipes that each bring a large flow of coolant water to the reactor. The coolant water is typically fed into the reactor from the outlets of heat exchangers used for steam generation. It is important to monitor the flow of coolant water through these pipes. If the coolant water flow drops significantly, the coolant temperature in the reactor would rise and the reactor could overheat, thereby damaging the reactor and possibly releasing large amounts of radioactive material into the coolant water. Therefore, water cooled nuclear reactors are typically shut down when the coolant flow is reduced below a predetermined value, typically about 90 percent of the normal level of coolant flow.
Most prior art water cooled reactor systems have elbows in the cold leg pipes carrying coolant water. These systems typically employ elbow taps for measurement of coolant water flow. In this arrangement, one tap is provided on an inner curvature of the elbow and another tap on an outer curvature of the elbow. Due to the dynamics of fluid flow through pipe elbows, the pressure measured by the outer curvature tap is generally higher than that measured by the inner curvature tap, and the difference in pressure between the two locations is indicative of the magnitude of flow through the pipe. A differential pressure transmitter operatively connected to each of the taps provides a mechanism for monitoring the flow through that elbow. In prior art reactor systems, a plurality of elbow tap pairs, wherein each pair is operatively connected to a dedicated differential pressure transmitter, are used to provide redundant measurements of the coolant water flow. When at least two out of three or four signals indicate a low flow condition, a trip signal is generated to shut down the reactor.
The Westinghouse AP600 plant configuration differs from prior art reactor systems in that there are no elbows in the cold leg pipes where elbow taps could be installed. Elbow taps installed on a large radius curvature in a cold leg pipe would not provide sufficient differential pressure for an effective measurement. In the Westinghouse AP600 plant configuration, there are two cold leg pipes connected to each steam generator, so differential pressure measurement across the steam generator tube bundle would indicate total flow through the two cold leg pipes but would not indicate individual loop flows. Therefore, an alternative flow measurement capability is needed for new reactor systems, such as the AP600, that do not have elbows in the cold leg pipes.